The present invention relates generally to an arrangement for improving the quality of a picture display system. In particular, the invention relates to an arrangement that is attached to a shadow mask and a method of manufacturing such an arrangement.
There is hardly any doubt that the problem of improving picture quality in color display systems, such as those utilized in domestic TV-sets, is of paramount importance and a focus of interest to a global visual media audience. As the quality of the systems for storing and providing signals representing visual media improves, the devices for reproducing the signals are obviously also required to improve in terms of their reproduction capacity. A typical example is the introduction of the digital versatile disc (DVD) and other digital image storage means that are now beginning to replace the video tape technology that has been used for several decades.
Regardless of the improvements that have been made in signal storage and processing technology, most display systems still utilize the technology of evacuated cathode ray tubes (CRT). Such systems comprise a multitude of electric components such as electron beam generating means, control circuitry and deflection coils as well as mechanical constructional elements such as a shadow mask. A shadow mask comprises a thin metal sheet having, for instance, a curved shape with a multitude of apertures through which the electron beams pass on their way to a phosphorus layer on the inner side of the display screen where a picture is obtained.
The shadow mask must remain mechanically stable in order to make sure that the electron beams reach the proper location, i.e. phosphor dot of the proper color, resulting in a picture having good color purity. Mechanical instability may occur due to externally induced vibrations as well as the buildup and release of mechanical stress due to heat transfer to and from the mask.
In order to attach the shadow mask to the CRT, while preserving a measure of mechanical stability, a prior art arrangement, U.S. Pat. No. 5,103,132, shows a mask that is attached to four frame parts in the form of stepped profiles. The stepped profiles are attached to each other so as to form a frame by means of connecting elements at the corners formed by the frame parts.
Although the arrangement shown in U.S. Pat. No. 5,103,132 may reduce negative effects relating to vibrations (microphony), the frame is not insensitive to negative effects caused by the release of mechanical stress inherent in the frame parts. Due to the fact that the frame parts are attached, e.g. by spot welds, to the curved skirt of the shadow mask, the frame will have inherent mechanical stress which may be released in an unpredictable manner when the mask is subject to external vibrations, shocks and thermally induced expansion and contraction.
An object of the present invention is to overcome problems related to prior art display arrangements as exemplified by the cited U.S. Pat. No. 5,103,132. In general terms, one problem to be solved is hence how an improved picture display system can be obtained. Another problem to be solved is how a display arrangement with improved resistance to thermal variations can be obtained.
The present invention provides a solution to these problems in that, according to a first aspect, it provides a display arrangement comprising a cathode ray tube, inside which tube a mask with at least one curved outer edge is located. The mask is at least partly attached by at least part of the curved edge to the tube by means of at least a diaphragm arrangement. The diaphragm arrangement comprises at least one diaphragm part in the form of an elongated folded sheet with a reduced level of in-plane mechanical stress. The sheet comprises a straight fold, and extends from the straight fold to a curved fold. The sheet further extends from the curved fold to a curved part of the sheet for attaching to the curved mask edge.
The invention further provides a solution to the problems stated in that it provides a diaphragm arrangement for use in such a display arrangement.
Yet an other aspect of the present invention provides a method of manufacturing a diaphragm part in the form of an elongated sheet, with a reduced level of in-plane mechanical stress, for use in such a display arrangement. The method comprises the steps of creating a straight fold and creating a curved fold such that the sheet extends from the straight fold to the curved fold, and such that the sheet extends from the curved fold to a curved part of the sheet for attaching to the curved mask edge.
Preferably, the straight fold has a varying folding angle and the curved fold follows a curve which enables the transition from the straight fold to the curved fold to provide a folded sheet which is free from inherent mechanical stress. The folds themselves will have a certain level of negligible inherent stress, but the sheet extending between the folds will be stress-free.
The diaphragm arrangement can thus be seen as an arrangement comprising three different sections: a curved section which follows the contour of the mask, a rectangular section which serves as a frame and a transition section between the curved section and the rectangular section. The function of the transition section is to act as a bridge between the curved section and the rectangular section.
An advantage of the invention is that it provides a stress-free diaphragm for holding the shadow mask. The implications for the function of the display system are that, by virtue of its dimensional accuracy, the mask and the diaphragm parts are more accurately fitted with respect to each other, and resistance to deformation during thermal cycling between low and high temperatures is obtained. This also means that there is a minimal deformation of the assembly of the mask and the diaphragm, which improves microphony damping and reduces the landing spread.
Apart from providing a stress-free diaphragm for holding the shadow mask, an arrangement according to the invention also has the advantage that it enables substantial freedom of design. The dimensions of the mask and the frame, i.e. the diaphragm, may differ significantly such that the mask may be either smaller or larger than the diaphragm. Although the mask may have virtually any reasonable curvature radius, it still allows a simple design of the diaphragm parts.
Another advantage of the present invention is that it allows a robust manufacturing method in that it is easy and provides a predictable end result. It is easy to reach the desired geometry in the folding steps. The resulting spread in the geometry of the manufactured parts is small and less dependent, as compared to prior art methods, on the pressure force used in the folding tool. In particular the inventive method enables a significant reduction to be achieved of the force needed to fold the parts, as compared to prior art methods, because of the fact that there is no deformation stress or strain in the sheet itself. This also means that the tool used for folding is less subject to degradation and hence requires less maintenance and repair, and said tool removes the need for hardened tools, and enables a large quantity of parts to be manufactured before being subject to degradation. This in turn allows easy manufacture or re-design of the folding tool, which means that the speed of a prototype-manufacturing process may be increased significantly and at a smaller cost as compared to previous methods where hardened tools must be used.